Conventional construction machinery (hydraulic excavator, wheel loader, etc.) comprising a hydraulic system for driving hydraulic actuators is generally equipped with a large engine selected in anticipation of maximum-load operations so that the construction machinery can deal with all types of operations from light-load operations to heavy-load operations. However, heavy-load operations are only a part of all the operations performed by construction machinery and such high engine performance can be too much or hard to handle when the load is light or medium. Thus, such a large engine is undesirable from the viewpoint of fuel efficiency.
Meanwhile, there exists a technology for reducing the fuel consumption (JP-11-2144-A). This technology reduces the fuel consumption by downsizing the engine to increase the fuel efficiency, constructing a hybrid system including a motor generator for compensating for the drop in the engine output caused by the downsizing of the engine, and operating the hybrid system at a high efficiency point by performing variable control on the rotational speed of the engine through the output assistance by the motor generator having quick output response.
In construction machinery comprising a hydraulic system, it is important to improve “workload fuel efficiency” (index representing how much the hydraulic actuators can work relative to the amount of supplied fuel). In order to improve the workload fuel efficiency, just operating the engine at its high efficiency point is insufficient; both the engine and the hydraulic pump have to be operated at operating points where excellent efficiency of the entire construction machinery is achieved.
A technology designed to resolve such a problem has been proposed in Patent Literature 2. In the technology described in JP-2009-74405-A, a target flow rate of the hydraulic pump is calculated by a target flow rate calculation unit based on the operator's lever operation for operating the hydraulic actuators. A target rotational speed of the engine (first target rotational speed) calculated according to the target flow rate of the hydraulic pump is compared with a target rotational speed calculated from the load pressure of the hydraulic pump and the lever inputs (fourth target rotational speed). The pump efficiency and the engine efficiency at times of high loads are enhanced by setting the final target rotational speed at the lower one selected from the first and fourth target rotational speeds.
In JP-2004-84470-A, the engine and the hydraulic pump are operated respectively at their high efficiency operating points by arranging a CVT (Continuously Variable Transmission) between the engine and the hydraulic pump.